The present invention relates to a method for making lithographic printing plates utilizing silver complex diffusion transfer process, and particularly to a method for making lithographic printing plate using coating development process.
Lithographic printing is carried out by feeding both water and ink to the surface of printing plates to allow the image portions to receive preferentially the coloring ink and the non-image portions to receive preferentially water and transferring the ink on the image portions onto a substrate such as paper.
Lithographic printing plates using silver complex diffusion transfer process (DTR process), particularly, those which have a silver halide emulsion layer and a physical development nuclei layer are disclosed in, for example, U.S. Pat. Nos. 3,728,114, 4,134,769, 4,160,670, 4,336,321, 4,501,811, 4,510,228, and 4,621,041.
The exposed silver halide crystals undergo chemical development in the emulsion layer by development treatment and become black silver to form hydrophilic non-image portions. On the other hand, the unexposed silver halide crystals are dissolved by the action of a silver halide complexing agent contained in a developer and diffuse to the surface physical development nuclei layer to be precipitated on the physical development nuclei as an ink-receptive image silver by the reducing action of developing agent.
The DTR process has such a mechanism that a chemical development and a dissolving physical development proceed simultaneously in a single development treatment. Therefore, in order to obtain good prints, important is balance of developing speed between the area where chemical development takes place to form a non-image portion and area where dissolving physical development takes place to form an image portion.
For example, if the dissolving physical development takes precedence over the chemical development, this causes stains on non-image portions or decrease in contrast of images, resulting in insufficient printing endurance. If the chemical development takes precedence over the physical development, this causes insufficient printing endurance.
Actually practiced processing method for making lithographic printing plates using DTR process generally employs an automatic process camera in which a developing tank is incorporated. That is, after exposure by the process camera, the plate is allowed to pass through a developing tank containing a large amount of a developer, and then the developer remaining on the plate surface is removed by a mechanical method, for example, allowing the plate to pass between squeeze rollers having a contact pressure. Then, the plate is allowed to pass through a neutralizing solution tank to adjust the pH of the plate surface, and the neutralizing solution remaining on the plate surface is removed by a mechanical method as in the case of removal of the developer.
According to the above processing method, amount of waste developer increases conspicuously in the case of processing a large quantity of plates, and maintenance or disposal of the waste developer is a great burden on users both environmentally and economically.
Furthermore, a lot of time is taken because the plate passes through the developing tank and the neutralizing solution tank, and exhaustion of developer such as decrease of pH and formation of sludge occurs owing to processing of a large number of plates.
As processing methods which solves these problems, JP-A-48-76603 and JP-A-57-115549, and U.S. Pat. No. 5,398,092 disclose a method of plate making in which a developer is fed onto a lithographic printing plate by coating the plate surface with the developer, and WO95/18400 discloses a processing method which comprises coating the plate surface with a developer by instantaneously dipping the plate in the developer contained in a very small developing tank. However, such coating development process suffers from various problems under the conditions different from those of practically employed dipping development process. These problems make it difficult to put the coating development process to practical use.
That is, in the above-mentioned coating development processes, fresh developer is not supplied to the plate surface after the plate surface has been coated with the developer, and if coating amount of the developer is decreased for reducing the amount of waste developer, the balance between the chemical development and the physical development is readily lost due to a slight deflection of the coating amount of the developer. As a result, the resulting printing plate is often stained in non-image portions or low in printing endurance.
In addition, a serious problem caused by decreasing the coating amount of developer is that temperature control of the developer is very difficult. For this reason, temperature of the developer is apt to lower and printing performance is extremely deteriorated. Thus, such a measure as using a roller for heating the plates or keeping the temperature around the developing part at constant has been adopted. In this case, however, water in the developer is evaporated and composition of the developer gradually changes, making the problems more serious.
According to a preferred embodiment of the coating development process, an excess amount of developer is previously fed to the photosensitive surface of the plate, a desired amount of the developer is coated on the photosensitive surface by a roller or blade, and an excess developer which has not been coated is recovered and reused. This is preferred from the points of uniform coating and reduction of the amount of waste developer. However, this coating development process has the problems that the developer contacts with air for a long period and pH of the developer tends to lower due to absorption of carbon dioxide in the air. Especially, pH of the developer greatly lowers in running processing in which a large number of plates are processed over several days, and the resulting printing plates are deteriorated in printing performance.
On the other hand, JP-A-5-127387 and U.S. Pat. Nos. 5,200,294 and 5,536,618 disclose use of a thioether compound alone or in combination with an alkanolamine in making lithographic printing plates using silver complex diffusion transfer process, but they do not mention to lower the temperature dependence of developer in the coating development process by using these compounds in combination.